In connection with the determination and/or monitoring of process parameters, there is a tendency increasingly in the direction of making available to the user, along with the measurement apparatus itself, also information on the mode of operation or the life span of the measurement apparatus under current system- and/or process-conditions. Catchwords in use in this connection are ‘Predictive Maintenance’ and ‘Mean Time Before Failure’. The goal of these efforts is ultimately to eliminate, or reduce to a minimum, the downtime of a measurement apparatus.